German Published Patent Application No. 102 44 553 describes an interferometric measuring device for recording the shape, the roughness or the distance of the surface of a measuring object, using a modulation interferometer to which short-coherent radiation is supplied by a radiation source and which has a first beam splitter for splitting the radiation supplied into a first beam component guided via a first arm, and into a second beam component guided via a second arm, of which the one is shifted with respect to the other with the aid of a modulating device in its light phase or light frequency, and passes through a lag line, and which are subsequently are united at an additional beam splitter of the modulation interferometer, having a measuring probe that is spatially separated from the modulating interferometer and is coupled to it or is able to be coupled to it via a light-conducting fiber set-up, in which the combined beam components are split into a measuring beam guided to the surface by a probe-optical fiber unit having a slantwise exit area on the object side and a reference beam, and in which the measuring beam (r1(t)) reflected at the surface and the reference beam (r2(t)) reflected at a reference plane are superposed, and having a receiver device and an evaluating unit for converting the radiation supplied to it into electrical signals and for evaluating the signals on the basis of a phase difference. In this context, the angle of inclination (y) of the exit surface to the normal of the optical probe axis amounts to at least 46°.
German Published Patent Application No. 198 08 273 also describes such an interferometric measuring device for recording the shape or the distance, especially of rough surfaces, using at least one spatially coherent beam generation unit, whose radiation is split up in a measuring probe into reference measuring beam guided through a measuring reference arm and reflected in it and into a measuring beam guided through a measuring arm and reflected at the rough surface, and having a device for modulating the light phase or for shifting the light frequency (heterodyne frequency) of a first beam component with respect to the light phase or the light frequency of a second beam component using a superposition unit for superposing the reflected measuring reference beam by the reflected measuring beam, having a beam decomposition unit and a beam receiving unit for splitting up the superposed radiation into at least two beams having different wavelengths and converting the radiation into electrical signals and having an evaluation device in which the shape and the distance of the rough surface is determinable based on a phase difference of the electrical signals. In this context, the radiation emitted by the radiation generating unit is temporally short coherent and broad-banded.
Such interferometric measuring devices, made up first of all of two interferometers may be constructed using different interferometer types. Thus, a modulation interferometer may be constructed as a Mach-Zehnder interferometer, while the measuring interferometer or the measuring probe is constructed compactly, for instance, as a Mirau interferometer. It is common to the two interferometric measuring devices that a path difference between two beam components registered in the first interferometer is balanced again in the second measuring interferometer or measuring probe by a short coherent radiation source, thus bringing the beam components to the formation of interference. The path difference registered by a delay element in German Published Patent Application No. 198 08 273 is also able to be generated, in this instance, by sub-arms of different lengths which are traversed by the beam components, as is shown in German Published Patent Application No. 198 08 273, in a modulation interferometer constructed using fiber optic guides.
In order to improve the measuring accuracy of the interferometric measuring device, it is known that one may connect a second output of the modulation interferometer to a reference interferometer. It is optically constructed in the same manner as the measuring interferometer, that is, it balances again the path difference between the two beam components registered in the modulation interferometer. The constructive design of the reference interferometer, however, differs from that of the measuring interferometer. The measuring accuracy of the interferometric measuring device may be improved by the comparison of the signals of the reference interferometer to those of the measuring interferometer.
The path difference that is to be registered in the modulation interferometer depends on the constructive execution of the measuring interferometer or the measuring probe. After a change in the measuring interferometer or the measuring probe, the path difference accordingly has to be correspondingly adjusted in the modulation interferometer. This generally takes place in a motorized manner, by shifting optical components.
In the reference interferometer the path difference also has to be adjusted correspondingly. In this instance, it is usual to adjust the path difference by substituting a preadjusted unit. The disadvantage of doing this is that adjusted units have to be available that correspond to the measuring interferometer being used. At the same time, the exchange of the units is costly.